tests: add mimic support in stable-3.1
tests: add mimic support in stable-3.1

Add mimic support in stable-3.1 branch so we can test it in nightlies CI
jobs.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

fgal/ceph-ansible,fgal/ceph-ansible